The conventional microwave oven existing in the market does not have a steam heating function. Although it has a high cooking speed and high cooking efficiency, but the cooked food is often lack of water, not fresh nor tender, therefore the conventional microwave oven is only suitable for limited types of food. On the other hand, the steam cookers existing in the market include steamers, electric saucepans, pressure cookers, even microwave steamers. However, all of these commercial available steam cookers can not generate superheated steam, and have defects such as low heating efficiency, low steam temperature, small amount of steam, large steam loss, low cooking speed, inconvenient operation, unsafe for use and unsatisfactory cooking effect. In a Chinese patent application publication CN1829882A, a steam cooker which can generate superheated steam is disclosed, wherein a sub cavity incorporating a steam heating heater is formed at the center of the ceiling part of a heating chamber. Steam in the heating chamber is sucked into an air blow device through a suction port and force-fed into the sub cavity through an external circulating passage. Gas passed through the external circulation passage sucks steam from a steam generator midway in the external circulation passage and enters from the external circulation passage into the sub cavity. The steam heated by the steam heating heater until it is brought into an superheated state is jetted downward from a plurality of upper gas jetting holes formed in the bottom panel of the sub cavity by such a force that makes the steam reach the bottom of the heating chamber so that the steam can be collided with a heated matter. The steam is raised on the outside of downward air flow and the convection of steam occurs in the heating chamber. However, in this patent application, an axial fan is adopted, and the circulation passage is very complicated, therefore a large space is needed; meanwhile, since the gas jetting holes are formed on the top of the heating chamber, the steam with higher temperature and pressure from the sub cavity blows downwards directly to the food from the top of the heating chamber, which causes the temperature of the top part of the food to increase rapidly, while the lateral part and bottom part of the food does not contact directly with the high temperature gas and the temperature thereof increases slowly. Therefore, during the whole heating process, the temperature within the heating chamber is not uniform, and the food can not be heated uniformly, causing some part of the food to be overheated but some part to be underheated, which affects the cooking quality.